1. Field of the Invention
The present invention relates to structured packing for vapor-liquid process towers and, more particularly, to a packing made up of corrugated, louvered contact plates disposed in face-to-face contact for use in such process towers.
2. History of the Prior Art
In the vapor-liquid contact art, it is highly desirable to utilize methods and apparatus that efficiently improve the quantity of the mass and/or heat transfer occurring in process towers. The technology of such process towers is replete with various packing designs used for tower packing. The types of packing employed are functions of the particular process to be effected within the tower. The packing elements may thus comprise a structured array (structured packing) arranged to form a regular array inside the column or may comprise relatively small shapes such as rings or saddles dumped into and randomly arranged (dump packing) within the tower. Close fractionation and/or separation of the feed stock constituents introduced into the tower and the elimination of harmful or undesirable residual elements imparts criticality to the particular vapor-liquid contact apparatus chosen for a given application. The shape of the dump or structured packing elements determines the flow patterns in and density of the array and the resultant resistance to flow caused thereby. Prior art structured packing arrays have thus found utility in a variety of shapes, sizes and material forms.
It has been found particularly desirable in the prior art to provide apparatus and methods affording efficient heat transfer, fluid vaporization, or vapor condensing whereby cooling of one of the fluids can be accomplished with a minimum pressure drop through a given zone of minimum dimensions. High efficiency, low pressure drop and reduced temperatures are important design criteria in the chemical engineering art such as petroleum fractionation operations. Process towers for effecting such operations ar generally of the character providing descending liquid flow from an upper portion of the tower and ascending vapor flow from a lower portion of the tower. Sufficient surface area for vapor-liquid contact is necessary for the primary function and the reduction or elimination of liquid entrainment present in the ascending vapor. Most often it is necessary for the structured packing array to have sufficient surface area in both its horizontal and vertical planes so that fractions of the heavy constituents are conducted downwardly in condensed form and that vapors are permitted to rise through the packing with minimum resistance. With such apparatus, heavy and light constituents of the feed are recovered at the bottom and top of the tower, respectively, by the interaction of the ascending vapor and descending liquid, mostly upon the surface of the structured packing.
A plurality of stacked layers affording compatible and complemental design configurations are generally assembled within a single process column. Each layer utilizes the velocity and kinetic energy of the ascending vapors to perform the dual function of eliminating liquid entrainment in the ascending vapor and the thorough and turbulent contacting of the vapor with the descending liquid to accomplish sufficient separation or fractionation of the fluids into desired components. Quick cooling of the ascending vapor is generally a prerequisite for efficient operation to effect efficient heat transfer for vapor condensation and minimum pressure drop in a minimum vertical depth of the packing. Oppositely inclined corrugated lamellae, or plates, have thus been utilized in the prior art for affording multiple vapor passages through the horizontal and vertical planes of the packing layers to insure the flow of vapor and distribution thereof within the lamellae and to prevent maldistribution, or channeling, of the vapor through certain portion of the layers and not others. Only in this manner is efficient and effective utilization of the column and the energy applied therein effected.
The structural configuration of inclined corrugated contact plates of the prior art variety often incorporates holes for vapor passage. Vapor turbulence is created by such holes to insure intimate vapor-liquid contact. It is also necessary to insure that the ascending vapor performs a dual function of liquid contact and liquid disentrainment within close proximity to the vertical location at which the ascending vapor approaches or leaves the vapor passage holes. In this manner maldistribution of the ascending vapor or descending liquid is reduced. It is, moreover, a paramount concern of the prior art to provide such methods and apparatus for vapor-liquid contact in a configuration of economical manufacture. Such considerations are necessary for cost effective operation.
Oppositely inclined corrugated plates provide but one method and apparatus for countercurrent, liquid-vapor interaction. With such packing arrays, the liquid introduced at or near the top of the column and withdrawn at the bottom is effectively engaged by vapor being introduced at or near the bottom of the column and withdrawn at the top. The critical feature in such methods and apparatus is to insure that the liquid and vapor achieve the desired degree of contact with each other so that the planned mass or energy transfer occurs at the designed rate. The internal structure is, of course, passive in the sense that it is not power driven externally and has few, if any, moving parts.
The prior art is replete with passive vapor-liquid contact devices utilizing cross-fluted and perforated sheets of material in face-to-face engagement. This configuration encourages the liquid moving through the packing to form itself into films having, in the aggregate, a large area over which the vapor may pass. However, the design problem is not merely a matter of providing a large surface area or a multitude of corrugations, cross-flutes, or perforations. A number of other interrelated design considerations must be taken into account, some of which have been mentioned above.
From a process standpoint, it is important that the desired vapor-liquid contact interaction be carried as close to completion as possible. For example, in a crude oil vacuum tower, efficient fractionation and good separation are needed to produce oil streams that are free of undesirable residual elements. As mentioned above, the contact column and its internal apparatus must efficiently utilize the heat supplied to the unit. In this manner, it minimizes direct operating costs, Whether the objective is mass transfer, heat transfer, liquid-vaporization or vapor condensing duty. With the above, pressure drop is a primary consideration as is the vapor-liquid fluid interface. Structured packing for vapor-liquid contact have been shown in the prior art in such references as U.S. Pat. No. 3,343,821, issued Sept. 26, 1967; U.S. Pat. No. 4,139,584, issued Feb. 13, 1979; U.S. Pat. No. 4,128,684, issued Dec. 5, 1978; U.S. Pat. No. 3,785,620, issued Jan. 15, 1974; and U.S. Pat. No. 3,959,419, issued May 25, 1976.
In the above-referenced vapor-liquid contact method and apparatus patents, several design configurations are presented for affording intimate vapor-liquid contact. In particular, stacked corrugated contact plates in face-to-face contact having corrugations inclined to the horizontal, and/or orthogonal one to the other, have been shown and provided in various material configurations. These configurations include monofilament yarns, and solid plates. It is moreover prominent in the prior art to utilize fluted plates having a plurality of perforations therethrough. One such example is seen in U.S. Pat. No. 4,296,050 issued Oct. 20, 1981, while another is seen in British Patent No. 1,004,046, published Sept. 8, 1965.
While many prior art methods and apparatus for vapor-liquid contact have been shown to be relatively effective, certain disadvantages still remain. In particular, vapor-liquid contact towers incorporating descending liquid flow and ascending vapor flow of the structured packing variety defined above, are generally incapable of readily accommodating internal pressure differentials. Problems also exist with surfaces that face downward, because such surfaces are generally not effectively wetted. Even with slits or lancing of the packing, there are many downward facing surfaces, and few prior art designs effectively address proper wetting or vapor passage therethrough. This is true even with a plurality of apertures disposed in corrugated and/or cross-fluted plates in face-to-face contact such as those referenced above. Vapor flow is ultimately sensitive to pressure differentials, and is easily diverted between the myriad of exposed areas of mating corrugations or flutes.
It is desirable, in countercurrent flow, that both the liquid and the vapor effectively commingle along uniformly wetted packing surfaces. In order for this to occur, it has been shown to be very beneficial for both the liquid and the vapor to be able to pass through the corrugated sheet for effective interaction. Without the free passage of both vapor and liquid through the sandwiched corrugated sheets, zones of either high or low volume flow can occur. These flow volume differentials result in a lack of uniformity and homogeneity within the packing. The most efficient structured packing configuration incorporates a region wherein the ratios of vapor and liquid remain relatively constant with consistent interaction and mixing. This requires a packing surface facilitating uniform flow of both liquid and vapor through both sides of the corrugated sheets, yet in a configuration promoting uniform wetting and spreading of liquid and equalization of pressure between said sheets.
It would be an advantage, therefore, to overcome the problems of the prior art by utilizing the flow directing and gathering features of louvers constructed in the corrugated plates. The methods and apparatus of the present invention provide such an improvement over the prior art packing by providing a corrugated plate having a select louver configuration therein. In this manner, liquid is caused to flow upon and through both sides of the corrugations of facing plates in paths which substantially increase the vapor-liquid contact of ascending vapor and descending liquid normally passing between said corrugated plates. The presence of selectively oriented arrays of louvers in the corrugated sheets permits vapor and liquid flows to be exposed on opposite sides thereof while flowing in opposed directions thereacross. Such liquid vapor flow configurations maximize mass transfer efficiency and may be provided with a minimal increase in production costs over that of conventional opposed plate corrugation assemblies.